Ohmic contact to semiconductor devices and method of manufacturing the same

ABSTRACT

An ohmic contact of semiconductor and its manufacturing method are disclosed. The present invention provides a low resistivity ohmic contact so as to improve the performance and reliability of the semiconductor device. This ohmic contact is formed by first coating a transition metal and a noble metal on a semiconductor material; then heat-treating the transition metal and the noble metal in an oxidizing environment to oxidize the transition metal. In other words, this ohmic contact primarily includes a transition metal oxide and a noble metal. The oxide in the film can be a single oxide, or a mixture of various oxides, or a solid solution of various oxides. The metal of the film can be a single metal, or various metals or an alloy thereof. The structure of the film can be a mixture or a laminate or multilayered including oxide and metal. The layer structure includes at least one oxide layer and one metal layer, in which at least one oxide layer is contacting to semiconductor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an ohmic contact to semiconductor devices andits manufacturing method, particularly an ohmic contact to p-typegallium nitride and the method of manufacturing the same.

2. Description of Prior Art

In recent years, gallium nitride (hereinafter referred to as GaN) hasbeen broadly used in the fabrication of short-wavelength light-emittingdiodes, laser diodes, photo-detectors and microelectronic components,etc. Good ohmic contact is especially important to commercializedlight-emitting devices. Currently, the specific contact resistance forn-type GaN has been reduced to about 10⁻⁴˜10⁻⁸ Ω·cm². As for p-type GaN,however, the specific contact resistance can only attain 10⁻²˜10⁻³Ω·cm², much higher than that for the contact to n-type GaN. Such a highinterface resistance markedly affects the performance and reliability ofthese devices. Therefore, it is an important issue for the scientistsand engineers to lower the specific contact resistance of the contact top-type GaN. Until now, most conventional methods to manufacture contactsto p-type GaN deposit the metals directly. For example, in U.S. Pat. No.5,652,434, the Nichia Chemical Industrial Company uses Ni or Ni/Au inits light-emitting diodes (LED) to form a contact. In addition, in U.S.Pat. No. 5,739,554, Cree Research Company uses Ti/Au, Ti/Ni or Ni/Au inits LED to form contact. But neither described the specific contactresistance of the contacts. In other references, other kinds of metalsare disclosed, such as Au, Ni, Ti, Pd, Pt, W, WSix, Ni/Au, Pt/Au, Cr/Au,Pd/Au, Au/Mg/Au, Pd/Pt/Au, Ni/Cr/Au, Ni/Pt/Au, Pt/Ni/Au, Ni/Au-Zn,Ni/Mg/Ni/Si, etc. However, the specific contact resistance of the abovemetal contacts can only attain 10⁻²˜10⁻³ cm-⁻², which is higher than10⁻⁴ cm⁻² generally required for optoelectronic devices. In addition,almost all of the above metals do not exhibit ohmic behavior.

SUMMARY OF THE INVENTION

Accordingly, the object of this invention is to provide an ohmic contactto semiconductor devices and its manufacturing method by which theinterface resistance of ohmic contact is lowered so as to improve theperformance and reliability of semiconductor devices.

This invention provides a new semiconductor manufacturing process whichcan form an ohmic contact to p-type GaN with a low interface resistancefor application in the fabrication of GaN-based devices.

The manufacturing method of this invention forms a film, which includestransition metal and noble metal, on the semiconductor substrate. Then,the film is heat-treated and oxidized to obtain an ohmic contact with alow specific contact resistance. So formed, an ohmic contact can meetthe requirement of an optoelectronic device; that is, the specificcontact resistance of the ohmic contact is lower than 10⁻⁴ cm-⁻².

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example and notintended to limit the invention solely to the embodiments describedherein, will best be understood in conjunction with the accompanyingdrawings in which:

FIG. 1 is a diagram illustrating the structure of an ohmic contactaccording to one embodiment of the invention;

FIG. 2 is a diagram illustrating the structure of an ohmic contactaccording to another embodiment of the invention;

FIG. 3a is diagram illustrating a pattern formed on a substrate in theCTLM measurement used in this invention;

FIG. 3b illustrates the current-voltage (I-V) measurement of Ni-Aucontacts formed on p-type GaN and heat-treated in various ambiences; and

FIG. 4 shows the specific contact resistance obtained by oxidizing Ni/Aulayers of different thickness.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of fabricating an ohmic contact to semiconductors accordingto this invention includes the steps of: coating a transition metal anda noble metal or an alloy thereof on a semiconductor material, thenheat-treating the metal layer in an oxidizing ambience so that thetransition metal is oxidized to form an oxide.

The semiconductor described above is p-type GaN. The transition metalcan be Ni, Mn, Cr, Cu, Fe, Co or Pd, etc. The noble metal can be Au, Pt,Rh, Ru, or Ir, etc.

The oxide mentioned above is a single oxide, or a mixture of variousoxides such as NiO/CoO or a solid solution of various oxides such asNi_(x)Col_(1−x)xO (O<×<1) etc. The metal in the above film can be asingle metal, or various metals or an alloy thereof.

Another layer of metal can be further formed thereon. Such layer ofmetal can a single metal such as Au or Ni, a plurality of layers ofmetals, or a layer of alloy such as Cr/Au or Ti/Pt/Au, etc., forconnecting with other circuits.

The ohmic contact formed by the above method has different structuresaccording to different preparation methods of the transition metal andthe noble metal. In the first embodiment, after heat-treatment, thetransition metal and the noble metal formed on the semiconductormaterial 10 becomes a mixture of semiconductor oxide 12 and metal 14 asshown in FIG. 1.

In the first embodiment, the above semiconductor material 10 is formedon a sapphire substrate, with an undoped GaN layer and a GaN layer dopedwith Mg, each 2 μm thick, formed by MOCVD method. Using thissemiconductor material as a test sheet, it is heat-treated in a nitrogenatmosphere to make the Mg doped GaN layer become p-type. This test sheethas an electron concentration of 1×10¹⁷ cm⁻³ for its undoped GaN layerand a hole concentration of 2×10¹⁷ cm⁻³ for its p-type GaN. A CTLM(circular transmission line model) method is used in the invention tocalculate the specific contact resistance (ρ_(c))

Next, the fabrication and measurement procedure for the ohmic contact ofthis invention is described, which includes the steps of: (i) forming aphotoresist layer on the GaN 20 with a CTLM pattern; (ii) removing theGaN surface oxide by dipping the test sheet in a solution of HCl:H₂O=1:1 for 3 minutes, then blowing dry the GaN and putting the testsheet immediately into a vacuum chamber of an electron-gun coatingsystem; (iii) degassing the chamber of the electron-gun coating systemto a high vacuum, then proceeding with the coating of various metals;(iv) lifting-off a part of metal film to form a metal pattern 22 asshown in FIG. 3(a); (v) heat-treating the test sheet in air, oxygen, 10%H₂-90% N₂ or nitrogen atmosphere, in which the temperature is from 200°C. to 900° C., and the time is 10 minutes; (vi) conducting I-Vmeasurement for the test sheet; and (vii) analyzing the ρ_(c) values.

Next, the CTLM measurement and analysis used in the above steps isdescribed, in which the measurement of I-V characteristic torespectively is used to figure out the resistance between the metalswithin the inner ring and outside the outer ring of two concentriccircles. The analysis of ρ_(c) is conducted on the I-V curves of ±0.5 Vand ±20 mA. Generally speaking, the contact structures of this inventionexhibits ohmic behavior within the above testing range, i.e., it isprovided with a linear I-V curve. Therefore, the specific contactresistance can be calculated through the slope of the curve. The formulaof calculating ρ_(c) for the CTLM method is as follows:

R _(t)=(R _(sh)/2π)[ln(R/r)+L _(t)(r ⁻¹ +R ⁻¹]

ρ_(c) =R _(sh) ×L _(t) ²

where R_(t) serves as the total resistance of the I-V measurement,R_(sh) is the sheet resistance, and r and R respectively represent theradius of the inner and outer concentric circles, L_(t) is the transferlength. According to the above formula, a diagram can be formed throughR_(t) of the I-V measurement to the ln(R/r). Then a linear curve can beobtained by processing the diagram with the least square linear curvefitting method. The slope of the obtained curve is R_(sh)/2π. Theintercept can thus be calculated by the formula when R equals to r, tobe R_(sh)L_(t)/rπ, so that R_(sh) and L_(t) can be figured out tofurther calculate ρ_(c).

FIG. 3b illustrates the measurement results of this invention, whichshows the I-V characteristic of Ni/Au contacts formed on p-type GaN andheat-treated in various atmospheres, wherein curve A represents thesituation in which Ni/Au is heat-treated in air or oxygen atmosphere,curve B, in nitrogen atmosphere, and curve C, in 10% H₂-90% N₂atmosphere. The temperature of the heat treatment process is 500° C. andthe heat-treating time is 10 minutes. The slope of the curve is amaximum, that is, the ρ_(c) value is a minimum, and the positive currentand the negative current is symmetrical to the original point afteroxidizing the Ni/Au film. On the other hand, the Ni/Au layer is still ametal film after the test sheet is heat-treated in nitrogen or 10%H₂-90% N₂. This results in an increase in the obtained ρ_(c). The I-Vcurve does not maintain linearity when the metal contact is biased at ahigher voltage, and the positive and negative currents are notsymmetrical to each other. Please also refer to the following Table 1,in which the Ni/Au thin film heat-treated in air of this embodimentstill displays a good conductivity.

TABLE 1 Condition Sheet resistance (Ω/□) Resistivity (μΩ · cm) As-deposited 11.87 17.8 N₂, 500° C., 10 min 16.82 25.2 Air, 500° C., 10 min38.94 97.4

FIG. 4 shows the specific contact resistance of the contacts formed byoxidizing Ni/Au layers of various thicknesses on the p-type GaN,wherein, curve A′ represents that Ni is 50 nm and Au is 125 nm, curve B′represents that Ni is 10 nm and Au is 25 nm, curve C′ represents that Niis 10 nm and Au is 5 nm. The oxidation of the above process is heatingthe test sheet in air for 10 minutes. According to the currentexperimental data, the minimum specific contact resistance is 1.0×10⁻⁴Ω·cm². Using X-ray diffraction to analyze the Ni(10 nm)/Au(5 nm) filmsheat-treated at 500° C. for 10 minutes, the result shows that Niconverts to NiO and Au is still metallic after heat-treated in air. Onthe contrary, when the test sheet is heat-treated in nitrogen or 10%H₂-90% N₂, the Ni/Au film is still metallic, but the ρ_(c) value isabout 10^(−1 to) 10⁻² Ω·cm². Furthermore, if instead of the above Ni(10nm)/Au(5 nm), a 50 nm thick Ni film is coated on the p-type GaN and thenthe same oxidation process is performed to form NiO, and the specificcontact resistance of the NiO contact to p-type GaN is measured toanalyze the effect of NiO, the ρ_(c) value is only about 0.1 Ω·cm², butits I-V curve is a linear curve over a wide range. This means that anohmic contact is formed between NiO and p-GaN. However, the ρ_(c) valueis high since the NiO thus formed is highly resistant. This indicatesthat the existence of NiO causes the oxidized Ni/Au film form an ohmiccontact. Au primarily gives the thin film with an excellentconductivity, because Au can not form an excellent ohmic contact top-type GaN. According to the prior art, it has been reported that ρ_(c)is only 53 Ω·cm² (L. L. Smith, et al, J. Mater. Res. 12, 2249(1997)) and2.6×10⁻² Ω·cm² (T. Mori et al., Appl. Phys. Lett. 69, 3537(1996)) for Aucontacts. It has also been reported that stoichiometric NiO isinsulating, but becomes p-type if doping with Li⁺ or creating Ni³⁺ ionvacancies in the NiO. Doping NiO with Li₂O can reduce its resistivity to0.1 Ω·cm (Z. M. Jarzebski, Oxide Semiconductors (Pergamon press, Oxford,1973), Chap. 10). Ni²⁺ ion vacancies formed during the oxidization of Nicreate holes (N. Birks and G. H. Meier, Introduction to High TemperatureOxidation of Metals (Edward Arnold, London, 1983), Chap. 4). Therefore,it is inferred that NiO formed in the oxidized Ni/Au is a p-typesemiconductor. Au and P-type NiO, which are in a condition of mixedmorphology, have a low interface resistance with P-type GaN and can forman ohmic contact to P-type GaN. Hence, Ni/Au film can form an ohmiccontact to p-type GaN after oxidation and heat-treatment, and isprovided with a low specific contact resistance.

According to the above inference, any thin film including p-typesemiconductor oxide and Au can form an excellent ohmic contact withp-type GaN. In addition to NiO, many oxides can be used to form a p-typesemiconductor such as MnO, FeO, Fe₂O₃, CoO (Z. M. Jarzebski, OxideSemiconducotrs (Pergamon press, Oxford, 1973), Chap. 11), PdO(R. Uriu etal., J. Phys. Soc. Jpn 60, 2479 (1991)), CuAlO₂(H. Kawazoe et al.,Nature 389, 939(1997)), SrCu₂O₂(A. Kudo et al., Appl. Phys. Lett. 73,220(1998)), Rh₂O₃(A. Roy and J. Ghose, Mater. Res. Bull 33, 547(1998)),CrO, Cr₂O₃, CrO₂, CuO, Cu₂O, SnO, Ag₂O, LaMnO₃, or YBa₂Cu₄O₈, etc.;therefore, it is also possible to form an ohmic contact to p-type GaNusing a mixture of this kind of oxide and Au. Furthermore, Au can bereplaced by other metals if the metal does not oxidize afterheat-treatment. Normally, any noble metal can be used, for example, Au,Pt, Rh, Ru, and Ir, etc.

Referring to FIG. 2, since the interface impedance of the p-typesemiconductor oxide and p-type GaN is very low, and the metal can forman ohmic contact having a low resistivity with the p-type semiconductoroxide, another embodiment of this invention comprises sequentiallyforming a layer of p-type semiconductor oxide 12 and a layer of metal 24on the p-type GaN 10 to form an ohmic contact to p-type GaN, such asp-GaN/p-NiO/Cr/Au, etc.

In the above embodiments, the ohmic contact to p-type GaN is described.However, the method of fabricating an ohmic contact can be applied inpractice to p-type Al_(x)Ga_(y)In₂N material, where 0≦X,4, Z≦1, andx+y+z 1.

In the past, the specific contact resistance of a contact formed onp-type GaN could attain only 10⁻²˜10⁻³ Ω·cm², but the ohmic contact ofthis invention can obtain a much lower interface resistivity of 1.0×10⁻⁴ Ω·cm². This improvement has been applied to the fabrication ofLEDs and GaN based laser diodes with good performance.

Furthermore, the metal formed on the semiconductor material in the lastembodiment can be replaced by a transparent conductive film, such asindium-tin oxide(ITO), ZnO or ZnO doped with Ga, In, Al or Ce, etc.

While the present invention has been particularly shown and describedwith reference to a preferred embodiment, it will be readily appreciatedby those of ordinary skill in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe invention. It is intended that the claims be interpreted to coverthe disclosed embodiment, those alternatives which have been discussedabove and all equivalents thereto.

What is claimed is:
 1. A method for manufacturing a low resistance ohmiccontact to a semiconductor comprising the steps of: providing asemiconductor substrate; forming a film including a transition metal anda noble metal on the semiconductor substrate; heat treating the film inan oxidizing environment to oxidize the transition metal to form ap-type semiconductor oxide, in which the film including the p-typesemiconductor oxide and the noble metal serve as an ohmic contact whichhas a specific contact resistance lower than 1×10⁻⁴ Ωcm².
 2. The methodfor manufacturing a low resistance ohmic contact to a semiconductoraccording to claim 1 comprising the further step of: the transitionmetal can transform into a p-type semiconductor oxide during the heattreating step.
 3. The method for manufacturing a low resistance ohmiccontact to a semiconductor according to claim 1 comprising the furtherstep of: the noble metal of the multi-layer film is selected from thegroup consisting of Au, Pt, Rh, Ru, and Ir.
 4. The method formanufacturing a low resistance ohmic contact to a semiconductoraccording to claim 1 comprising the further step of: wherein the filmformed on the semiconductor substrate can be an alloy of transitionmetal and noble metal.
 5. The method for manufacturing a low resistanceohmic contact to a semiconductor according to claim 1 comprising thefurther step of: wherein the transition metal is selected from the groupconsisting of Ni, Mn, Fe, Co, Cr, Cu, and Pd.
 6. The method formanufacturing a low resistance ohmic contact to a semiconductoraccording to claim 1 wherein the step of: providing a semiconductorsubstrate includes the step of providing a GaN semiconductor.
 7. Themethod for manufacturing a low resistance ohmic contact to asemiconductor according to claim 6 comprising the further step of:providing a p-type GaN semiconductor includes providing a p-typeAl_(x)Ga_(y)In_(z)N semiconductor wherein 0≦x, y, z≦1 and x+y+z=1.